C++ : Remove an overzealous checking assert [PR97871]

C++ : Remove an overzealous checking assert [PR97871]

It seems we accept __attribute__(()) without any diagnostic at present,
so my added checking assert fires for something like:

__attribute__ (()) int a;

Fixed by removing the assert; in the case that the user enters something
like:

__attribute__ (()) extern "C" int foo;

The diagnostic about attributes before linkage specs will fire and show
the empty attributes.

gcc/cp/ChangeLog:

	PR c++/97871
	* parser.c (cp_parser_declaration): Remove checking assert.